It is common practice to mix particulate solids, liquids and gases with motionless mixers having, as the name implies, no moving parts. Mixers of this category consist of baffles of various types arranged sequentially in a tube or pipe. By a process of division and recombination, separate input components can be mixed or dispersed within one another at the output of said tube or pipe.
Difficulties are often experienced, however, when mixing materials of widely disparate viscosities and/or very different flow rates. For example, in the polymer field, it is at times desirable to mix very small quantities of a low viscosity material within a much larger quantity of a high viscosity material. When this is done, the low viscosity material tends to tunnel through the mixing elements without blending with the high viscosity material to any great extent. As an example, one might wish to mix a stream flowing at a rate of 7 gpm of a polymer having a viscosity of 30 million centipoises with a second stream traveling at 0.035 gpm of 6 centipoise material.
A variety of approaches have been attempted to produce an initial degree of dispersion or mixing at the injection point of the low viscosity material. These approaches have included, by way of illustration, the use of a multiplicity of injection ports around the circumference of a pipe. A second approach has consisted of the use of a relatively small diameter pipe for carrying the low viscosity material which passes through the diameter of the main pipe carrying the high viscosity material. The small diameter pipe is configured to have a plurality of holes used for injecting the low viscosity fluid. A common problem of such devices having parallel path outlets is that the low viscosity fluid injection apertures become differentially plugged resulting in asymmetric distribution.
It is well known that one of the mechanisms that allows for mixing of fluids is diffusion. However, when dealing with high viscosity materials which typically produce laminar flow, diffusion rates are very small. It is known that the rate of mass transfer N of the diffusion component measured in moles per second per unit area is equal to the diffusivity D multiplied by the local concentration gradient dC/dr. Thus, EQU N=D(dC/dr)
Since D is small in high viscosity material, it is necessary to make the concentration gradient dC/dr large in order to maximize the value of the mass transfer rate N.
As being typical of a difficult mixing system is the development of a continuous polymerization of methyl methacrylate to produce the acrylic resin. This requires the introduction and intermolecular mixing of less than about one percent of a very low viscosity additive, about 6 cp, and to a high viscosity melt system. The latter viscosity is about 15 million cp at the operating shear rate. The problem is exacerbated by the desire to minimize thermal and mechanical abuse to the final product. First thought is given to the use of in-line or motionless, also called static, mixers. Although they represent savings in power and capital investment, when introducing a low viscosity liquid in to a high viscosity process stream, motionless mixers tend to be ineffective allowing the low viscosity liquid to simply tunnel through the high volume, high viscosity fluid.
Others have suggested the use of compounding extruders to mix additives into the polymer. This introduces heat, shear history and high energy costs. It was found, however, that perhaps a static mixer could be used if the appropriate entrance conditions were met. This recognition in and of itself, represents a rather significant departure from prior teachings which tend to encourage the use of static or motionless mixers only in turbulent flow conditions. When engaging in the polymerization of methyl methacrylate, the flow is highly laminar with a Reynolds number of approximately 10.sup.-4. As a solution to this problem, a distribution head was devised which, when used in conjunction with a static or motionless mixer, provides an effective mixing element principally due to the "sheeting" of the low viscosity additive prior to the introduction of the process stream into the motionless mixer element.
The device disclosed in applicant's U.S. Pat. No. 4,808,007 has been successfully applied to a wide variety of additive mixing problems. The device causes an additive to be introduced to the main fluid flow within a pipe or conduit in the form of a thin sheet. This generate a large interfacial area "A" in combination with a small interfacial thickness "t". It will be understood that a large A/t value is desirable in order to assist molecular diffusion and therefor mixing of the additive within the main product flow. The device is usually used in combination with a downstream static mixer to complete the mixing task.
The device disclosed in the '007 patent has been particularly useful in industrial manufacturing processes for a wide variety of plastics such as polyethylene, polystyrene and polymethylmethacrylate. In the production processing of such plastics, the melt viscosity of the polymer is typically in the range of 5.times.10.sup.5 to 10.times.10.sup.6 cp. The additives to be introduced and mixed within the main polymer are wide-ranging in type and properties. They include colorants, catalysts and lubricants. These have viscosities in the typical range of 0.5 to 100 cp.
It is understood that other physical properties, such as molecular diffusion rates, surface tension and specific gravity can affect the operation of the device. In some cases, for example, the additive is found to readily wet the outer walls of the conduit. Since low viscosity material will generally seek the highest shear rate at the conduit wall, the generation of additive sheet is compromised.
It is thus an object of the present invention to improve upon the invention disclosed in the '007 patent by reducing the tendency of the additive to wet the walls of the conduit while reducing any negative impact which such an improvement would have upon the A/t value discussed above. These and other objects of the present invention will be more readily apparent when considering the following disclosure and appended claims.